Dr Igor Sutyagin, of the Royal United Services Institute, said: “These separatists boasted on Twitter about capturing a BUK SA11 missile launcher on June 29, and several hours before the downing of the plane locals in Torez reported seeing BUK missile launchers and separatist flags around the city.”

He added there was also a suggestion the launcher was being manned by soldiers from Russia.

The downed passenger plane is thought to have been mistaken by rebels for a Ukrainian government resupply flight.

Ukrainian security services claim to have intercepted two phone conversations in which pro-Russian separatists appear to admit to shooting down Flight MH17.